1. Field of the Invention
Apparatuses consistent with the present invention relate to reproducing sound signals which are carried over ultrasonic waves, and in particular, to a sound reproducing screen for an ultrasonic converting and reproducing method.
2. Description of the Related Art
Among methods of transferring sound signals over a long distance, methods of transmitting sound signals carried on ultrasonic waves have been developed. Such methods were first developed for military purposes such as a submarine, and such methods have led development of speakers for home and industry in recent years.
The term “ultrasonic wave” means a sound wave having a frequency of 20 kHz or more, which is higher than an audible frequency. When a sound signal is carried on an ultrasonic wave and then transferred, a sound source can be obtained which has a stronger intensity and which has directivity.
A speaker which uses the conventional properties of the ultrasonic wave, usually employs a method of amplitude-modulating the sound wave to the ultrasonic wave. An output signal of such speaker is converted to a sound signal in an audible band which can be heard by a person during its transferring procedure by the non-linearity of a medium.
The conventional ultrasonic sound source can send a sound wave over a long distance, or can make a sound wave directed, to a specific point. However, only a small portion of acoustic power of the sound signal carried on the ultrasonic wave is actually transferred, so that a strong output must be used as compared to the typical speaker. Accordingly, a listener is exposed to a portion of a strong ultrasonic wave field. It is thus not suitable as a sound source for a listener as compared to the conventional speaker which only generates a sound signal in the typical audible band.